Two-Electron Charge Separation in Benzo[ghi]perylene-TCNQ Charge-Transfer Complexes

High Resolution Image Download MS PowerPoint Slide Organic charge-transfer complexes (CTCs) provide a versatile platform for precisely controlling electron donor–acceptor packing and electronic coupling. It also conveys insights into how ground-state charge-transfer interactions influence photoinduced excited-state dynamics. Here, we investigate crystalline complexes of electron-donating benzo[ghi]perylene (BP) and electron-accepting 2,2′-(2,5-cyclohexadiene-1,4-diylidene)dimalononitrile (TCNQ), which exhibit pronounced charge-transfer character and broad absorptions across the visible region. Upon selective photoexcitation into the charge-transfer band, femtosecond transient absorption spectroscopy reveals the ultrafast evolution of an initially photoexcited state into a charge-separated configuration featuring [BP 2+ ···TCNQ 2– ] spectroscopic fingerprints within 1.6 ps. Global analysis resolves two sequential species that capture the progression from charge localization to a stabilized state, followed by geminate recombination to the ground state on a 12–13 ps time scale. Fluence-dependent measurements show linear signal scaling and excitation-independent kinetics, confirming first-order excited-state dynamics and excluding higher-order excitonic contributions. Consistently, the absence of a long-lived species indicates that charge carriers remain Coulombically bound within the crystalline lattice. Together, these results reveal how strong intermolecular coupling and orbital energetics govern the formation of excited charge-separated states in electron donor–acceptor CTCs.